3.3v logic level mosfets

Do you need mosfets or would a darlington due. With a beta around 5000 it would only take 200uA to drive a 1A load.

What's wrong with the 2n7000,too big? Use a smaller BSS138 in a sot-23 package.

2n7000 is fine but Its only 60v rated, I need about 200v, ... 150v + some inductive overshoot. darlington is an idea wich I'd not thought of, but its for a low power inverter running from the 5v so low fwd drop is preferable. the somewhat complex pwm control is done from a PIC. the 2n7000 is for -20v inverter.

I think i might have to go for a driver and some beefier mosfets as I cant get enough power from the ones I have even with 5v pullups, its not quite as low power as id hoped lol. Il have to bring in 12 for the driver I can run the inverter off 12v instead of 5 too.

However I did have a look at fairchild's site and its very nice to use, found one that I might try anyway, thats if I can get hold of it - Fqt4n20. although looking for that one in Farnell i came across a bsp297or zvn2120G but the later is no longer stocked, They didnt stand out in previous searches.

thanks, Colin =^.^=

how bout this one.

Needs way more than 3.3V drive unless you want to cook it. Vgs(th) is 3V max, they spec it at 10Vgs to really turn it on, 5V will turn it on some but

3.3V and it's really gonna be a bar-b-q with any current.

I'm thinking maybe the OP should look into IGBTs. I don't know about the

3.3V drive, but it's way easy to find IGBTs that will withstand 600Vds and switch large currents. The down side is that giant cap sitting on the gate(base). As long as you can dump the juice in fast enough, they're slick. Of course I only played with them once and I used 5V drive. Mr Hill is the man to ask about them.

That vn2224 looks realy cool, maybe I can add an offset to the gate drive of

1volts, that will give me 4.3 volts drive and still be sure its off, maybe set the vdd to the max of 3.6 volts and/or go for 1.2 volts, gate offset. the open drain 5v pull up isnt so nice when programming the device and the outputs go tri state.

the graph shows at 1 amp its saturated at vgs=3v., I hadnt intended to need more than 1amp, its an inverter so the current is ramped from 0-1amp, average 50% duty cycle, so 0.25A avg I only need about 1.5W real power output from the device im driving, although it seems I need a fair bit more due to reactive currents in the load, at 4vgs its over 4amps wich should be ample for my needs, typical ofc.

Theres a quad package too, maybe 2 in parallell will easily do, or I can use the other 2 to save a discrete elsewhere.

As for devices with large input capacitance such as igbt, the current from my PIC is somewhat limited, so switching speed will become unaceptable so I would need a driver anyway wich then makes the fet selection easy anyway. the output from the inverter is 1/2 sine wich is why its PWM is driven from the PIC. it should drive the 2nc gate charge of the vn2222 quick enough.

Id rather drive everything from the 5v but its a bit of a jump from 5v to

150pk and demands a bigger device but the 12v supply is via a linear regulator, the 5v via a switcher.

Unfortunatly the VN2222 that is stocked by RS and farnell seem to be 60v spec to92 devices. seems to be some confusion with part numbers here >.< neither stock the vn2224 quad. is the 200v version an unobtanium part ?

Think I might just plop for a driver and be done with it. I just tried a dual driver I had available but cldnt get it to work, then reading the data sheet realised the shutdown input had a threshold of 11v lol.

thanks Colin =^.^=

If I understand what I'm reading, Vds will be 1V at 3Vgs and 1A, that's a dissipation of 1W. It's gonna get real hot if you draw an amp thru it.

Am I not reading this right? At Vgs=4V and Id = 4A, I come up with a Vds of

5V. Isn't that 20W of dissipation?

I agree that the PIC should have no trouble charging the gate of that device.

All I know is that I couldn't find any FET that I could afford/use at 400V, I had to use an IGBT and a gate driver:

TC4423 by microchip $2 in singles at Digikey. Good to 18V on the output and should work fine at 3.3V.

4423 dual inverting 4424 dual non-inverting 4425 dual one-of-each

yea those dissipations are quite high, its a current ramp though so only the tail end power will be that high, the average will work out a lot lower. the on resistance would of course severly limit operation to less than 4amp as id rather use 5v for the inverter. At 12v i would need less current too ofc.

Well depends how fast you want to switch it, 100khz would be ideal. the dspic33 is a bit weedy on the io pin current side. rated 4ma but seems to easily sink a peak of 10ma.

I had a dual SO8 500v mosfet wich i used as a class d stage, fed by an inverter. Im trying to simplify it drastically by doing away with the inverter and the hi and low side driver.

dual low side drivers should be cheaper than that I hope, I dont need 3amp gate drive, thats as much as drain current, or discrete will do.

thanks Colin =^.^=

Does pullup to 5V really help any? won't the voltage just get clamped by the output to something close to 3.3V?

-Lasse

Ok, most of the ones I tinker with will supply 20mA.

Those are for fairly healthy sized MOSFETs and IGBTs. Microchip's AN786 app note might interest you. It has some good info on how to "correctly" calculate the amount of drive current you will really need to switch in a certain amount of time. Those microchip gate drivers above will switch in

30nS or better. I guess it all depends upon what you need. :-) It's about PWM'ng power supplies that have high and low side drive, but might interest you anyway.

the particular PIC outputs can be set to open drain and then have a 5v tolerance.

Colin =^.^=

If your present mosfet drives OK, and you really care about high voltage, consider a cascode. Either another MOSFET, or (I'd prefer this) an NPN transistor, with base connected to your 3.3V rail, should fit the bill. Connect the base through a parallel RC to get lower ON voltage.

It's not a perfect switch-to-ground, but it's quite easy to drive.

yes a cascode arangment that was something I had considered as an alternative to a driver. and a 2 mosfet cascode needs no extra resistors.

Colin =^.^=